Transcript Document

Is Baja California part of the Pacific Plate?
Ch. Plattner1 R. Malservisi1,2
T. Dixon2 P. LaFemina2 G. Schmalzle2 J. Fletcher3 F. Suarez-Vidal3
1
Coupling GPS observations with numerical modelling we want to constrain the kinematics of Baja California. Here we present
results from data interpretation in a rigid plate model.
2
3
1.1 Purpose:
Ludwig-Maximilians University, Munich, Germany
RSMAS, Miami FL
CICESE, Ensenada, Mexico
T33B-055
4. Velocity of Baja California in stable Pacific plate reference frame:
Plate boundaries are tectonic active regions at which the motion of the interacting plates has to be accomplished. On continental lithosphere theses regions appear as wide
spread deforming zones. Using geodetic data from GPS observations we can observe the recent motion and deformation of these zones and estimate the coupling between
the interacting plates. Understanding the kinematics of the tectonic processes we can also estimate the dynamic effects implicated in the surrounding regions. The motion of
Baja California peninsula is a result of the rupture of this terrane from the continental lithosphere of North America, thus it is constraining the spreading of the Gulf of
California.
Fig. 9, 10: Velocity at sites in northern and
southern Baja California with respect to stable
Pacific plate. Error ellipses represent a 95%
confidence interval. Epicenters of earthquakes
since the 1970s are marked by red dots (NEIC),
focal solutions are from CMT Harvard.
1.2 Test site:
Fig. 11: Comparison of these residuals to
Nuvel1A PA(NA) velocity indicates that Baja
California is moving with this direction slower
than Pacific plate. The residuals vary by
magnitude and direction. Note that the majority
of the seismicity is related to the spreading of
the Gulf.
Fig. 1: Formation of the
North American/Pacific
plate boundary and the
Baja
California
T.
Atwater
http://emvc.geol.ucsb.e
du/
Until 12 Mio a. Baja California was part of the North American plate. The subduction of the Farallon plate stopped with the mid oceanic ridge reaching North America. The
new plate boundary of North America and Pacific plate had to accommodate to the strike-slip motion and the San Andreas system established. Also distributed extension and
translation of terranes took place at the western margin of North America. The continental lithosphere of Baja California ruptured about 5 Myr ago. Seafloor spreading in the
Gulf of California started at 3.5 Mio a.in the southern basins of the Gulf. Since then Baja is transported with the northwest moving Pacific plate and is as a terrane part of this
diffuse plate boundary between Pacific and North America just like the Basin and Range and Sierra Nevada block to the north. Is this transfer complete or is Baja California
not fully coupled with the Pacific plate? Using GPS velocity fields we want to answer this questions.
5. A rigid plate model for Baja California:
2. GPS data analysis
GPS data processing at the University of Miami using GIPSY/OASIS code from the JPL/NASA
GPS campaign data from CICESE, University of Miami
Southern California Earthquake Center for Baja California and
data from IGS permanent sites for Pacific and North American
plate.
(Dixon et al. 1997, Sella et al. 2002).
 Daily solutions and error estimates in IGSb00 reference frame. Velocity at sites calculated
from daily solutions by a weighted least squares regression. Error estimates according to Mao et
al. 1999.
Fig. 12: Extension of the rigid Baja California
block defined by the Agua Blance fault (ABF),
San Pedro Martir (SPM), the spreading center
and transform faults in the Gulf of California,
the La Paz fault system, the Tosco Abreojos
fault zone (TA) offshore to the west of Baja
California.
Sites north of the rigid block show residuals that
can be related to the dense network of active faults
which can also be seen from seismicity (fig. 12).
Compatible with the normal fault system at the
Sierra La Laguna, the Cabo region south of the La
Paz fault system is not moving with the central Baja
block. This region shows a complex pattern of
deformation probabaly related to the earthquake
cycle effect (fig. 13).
Sites used for pole of rotation: CADG, COLO, CONC,
LOSA, MELR, SAIS and SLRE. Sites within the rigid
block not used for the rotation pole calculation (fig.
12) are: AGUA and WCAR near active Loretto basin
and CARD near La Paz faults system probably
reflecting strain accumulation or earthquake cycle
effect.
Fig. 13, 14: Velocity at sites in northern and
southern Baja California with respect to the rigid
Baja California block. Error ellipses represent a
95% confidence interval.
ABF
SPM
WCAR
AGUA
TA
LPFS
Fig. 2-5: Daily solutions and error
estimates in IGSb00 and velocity [mm/yr]
for sites on northern (2-3) and southern
(4-5) Baja California
CARD
3. Stable plate reference frames:
Fig.
6a
and
6b:
Residuals of rigid plate
motion for Pacific plate
(a) and North American
plate (b) computed
from
the
particular
Eulerian
Pole
of
rotation. Error ellipses
represent
a
95%
confidence interval.
1
Omiga
[deg/Myr]
BC(IGSb00)
Omiga [deg/Myr]
An Eulerian pole of rotation with an angular velocity ω describes a rigid plate motion on the sphere. Deformation at the plate boundary is not
considered due to this rigidity assumption. The best fit Eulerian pole of rotation is calculated from observed IGSb00 velocity from site selected to be
on the rigid plate by minimizing residuals (chi²). As the pole of rotation and its angular velocity is calculated in ISGb00 reference frame we can
directly compare its angular velocity to the observed velocity at any GPS site.
The Eulerian pole acts like a transformation to go from IGSb00 into a Pacific plate reference frame, so that we can estimate the difference of the
observed GPS velocity at sites on Baja California to the angular velocity from the stable Pacific plate Eulerian pole.
Fig. 7: Location of rotation
poles and error ellipses
representing 1 standard
deviation. Poles for PA from
this work and Beavan et al.
2002 are within error the
same.
Fig. 8: Comparison of
magnitude of Eulerian poles
from this work.
6. Conclusion:
PA(IGSb00)
PA(ITRF2000)
PA(NA)1
NA(BC)
PA(NA)
PA(BC)
0.8
NA(PA)
PA(IGSb00)
0.6
NA(BC)
BC(IGSb00)
0.4
0.2
The central part of Baja California moves
as a rigid block.
PA(BC)
0
5mm/yr
Table 1: Listing of Eulerian
poles of rotation, if not
other stated poles are
results from this work and
based on geodetic data.
From a rigid plate model we cannot
describe the motion of Baja California by
the Pacific pole of rotation. The residuals
at Baja vary between 3-5 mm/yr, indicating
that Baja is not moving with the full Pacific
rate. Thus the Gulf of California does not
accommodate the full PA-NA motion.
Pole of rotation (reference frame)
Longitude [deg]
Latitude [deg]
Omiga + Sigma [deg/Myr]
Major and minor Sigma [deg]
Azimuth [deg]
PA(IGSb00)
111.36
-63.74
0.6797 + 0.0025
0.30/0.16
89.7
NA(IGSb00)
-84.39
-5.53
0.1945 + 0.0018
0.63/0.18
-1.3
BC(IGSb00)
111.80
-66.32
0.5903 + 0.0422
4.23/0.24
-61.9
PA(BC)
-70.31
47.28
0.0938 + 0.0475
23.15/2.11
79.8
NA(BC)
-75.36
50.71
0.6736 + 0.0490
3.04/0.23
70.4
NA(PA)
-74.70
50.30
0.7671 + 0.0035
0.28/0.17
82.4
PA(NA) Nuvel1A (geological)
-78.2
48.7
0.75 + 0.01
1.3/1.2
119
PA(NA)1 Beavan et al. (2002)
284.96
50.26
0.773 + 0.005
0.41/0.17
94
The Eulerian vectors confirm that Baja
California is moving with respect to the
Pacific plate.
The velocity at sites north of the Agua
Blanca fault and in the Cabo region is
influenced of nearby faults, their seismicity
and earthquake cycle.